Hi,

I have aquired an old 500VA computer UPS from my friend which was unused for a pretty long time.. After setting in a battery and charging it, the UPS works well but I have noted that the switching of the UPS back to mains from battery when the power returns results in a sudden spike for a millisecond during the time which the relay switches.. I noticed this when I was testing it with a 60W 240V incandascent bulb.. Except this everything else is fine.. Is this normal for a computer UPS?? During the spike, the bulb blinks with high brightness and returns back to normal and I tried it many times but the bulb seems to have no problem...
This UPS was used with an old PC with an AT power supply.. Will this iregularity cause a problem of I connect it to my PC with an ATX supply? How can I solve it or is it normal??

 

Will a capacitor over the switching relay solve the problem??

 

If that's how it was built, it should be OK - but I'd connect the computer via a surge protector, just to be on the safe side..

 

But I was wondering if it was just a relay after that step-up transformer, what could cause such a spike??